1. Field of the Invention
This invention relates to a startup method for an olefinic naphtha hydrorefining process.
2. Description of the Prior Art
Hydrorefining is a well known process for upgrading a variety of hydrocarbon fractions. Naphtha fractions are subjected to hydrorefining by treatment with hydrogen and a hydrofining catalyst to remove sulfur and nitrogenous compounds which may adversely affect the catalyst used in subsequent treating processes, such as, for example, reforming processes. When the naphtha fraction contains olefins, such as, for example, cracked naphthas or coker napthas, at least a portion of the olefins are hydrogenated to saturated hydrocarbons during the hydrorefining operation. Since the olefins are high octane components, for some end uses of the naphtha, it is desirable to retain the olefins rather than to convert them to saturated compounds. Conventional fresh hydrorefining catalyst have both hydrogenation and desulfurization activity. Typically, a fresh or a regenerated catalyst is used at the start of the hydrorefining process.
It is known to presulfide hydrorefining catalysts with a sulfur-containing hydrocarbonaceous oil such as a sulfur-containing naphtha prior to the hydrorefining step.
It is known to hydrodesulfurize a catalytically cracked naphtha under mild hydrodesulfurization conditions, including a pressure ranging from about 200 to 700 psig and in the presence of a partially spent catalyst to minimize hydrogenation of olefinic and aromatic constituents (see, for example, U.S. Pat. No. 2,983,669, column 6, lines 10-19).
It is also known to hydrodesulfurize an olefinic gasoline by adding a nitrogen compound to the feed in order to deactivate the catalyst for the hydrodesulfurization reaction (see, for example, U.S. Pat. No. 2,913,405).
It is known to hydrotreat a straight run fuel oil at a pressure not greater than 150 psig and at a temperature from 400.degree. to below 500.degree. F. in the presence of a catalyst that had been employed in a prior hydrotreating process operated at a higher pressure than the pressure of the fuel oil hydrotreating step until the catalyst had been substantially permanently deactivated for the purpose of said high pressure process, as shown in U.S. Pat. No. 3,870,626.
It has now been found that a hydrorefining process can be conducted with a minimal conversion of olefins to saturated hydrocarbons when a partially deactivated hydrorefining catalyst is used at the start of the run instead of utilizing fresh catalyst and that the pressure in the hydrorefining zone is maintained below about 200 psig.